Alkali resistant optical coatings for alkali lasers and methods of production thereof

ABSTRACT

In one embodiment, a multilayer dielectric coating for use in an alkali laser includes two or more alternating layers of high and low refractive index materials, wherein an innermost layer includes a thicker, &gt;500 nm, and dense, &gt;97% of theoretical, layer of at least one of: alumina, zirconia, and hafnia for protecting subsequent layers of the two or more alternating layers of high and low index dielectric materials from alkali attack. In another embodiment, a method for forming an alkali resistant coating includes forming a first oxide material above a substrate and forming a second oxide material above the first oxide material to form a multilayer dielectric coating, wherein the second oxide material is on a side of the multilayer dielectric coating for contacting an alkali.

The United States Government has rights in this invention pursuant toContract No. DE-AC52-07NA27344 between the United States Department ofEnergy and Lawrence Livermore National Security, LLC for the operationof Lawrence Livermore National Laboratory.

FIELD OF THE INVENTION

The present invention relates to alkali vapor resistant coatings, andmore particularly, to alkali vapor resistant optical coatings for alkalilasers.

BACKGROUND

Lasers utilizing an alkali vapor are relatively new. In these lasers,the alkali atoms are excited from the ground ²S_(1/2) state to theexcited ²P_(3/2) state by absorption of laser diode pump light and lasedfrom the ²P_(1/2) state that lies just below the pumped level.Non-radiative interstate relaxation between the ²P_(3/2) and the²P_(1/2) state may be achieved through collisional deexcitation orenergy transfer to a molecule, for example ethane. Because the lasingenergy photon is just slightly less than the exciting photon, theselasers may be very efficient. Also, the fact that these lasers use a lowdensity vapor means that there may be little wave front distortion.

Alkali lasers have been made using potassium, rubidium, and cesiumvapors. As shown in FIG. 1, which shows the energy levels involved inlasing a rubidium line in an alkali laser, the pump photon energy (780nm) is just slightly higher than the lasing photon energy (795 nm). Theuse of a vapor requires either that the vapor be of a fairly highconcentration or that a reasonably long path length be provided toabsorb all of the pump radiation. However, because these alkalis arevery reactive materials which are commonly used for reflective andantireflective surfaces within lasers are susceptible to alkali attack.,and therefore are generally poor choices for use in alkali lasers andother devices which use alkali gas.

Therefore, it would be very beneficial to have an alkali resistantmaterial and/or coating capable of being used in alkali lasers and otherdevices which use alkali gas that will not be chemically attacked orallow the alkali to diffuse, but also is highly reflective or lowlyreflective.

SUMMARY

In one embodiment, a multilayer dielectric coating for use in a windowor reflector of an alkali laser includes two or more alternating layersof high and low index dielectric materials, wherein an innermost layerof the two or more alternating layers of high and low index dielectricmaterials includes at least one of: alumina, zirconia, and hafnia forprotecting subsequent layers of the two or more alternating layers ofhigh and low index dielectric materials from alkali attack.

In another embodiment, a method for forming an alkali resistant coatingincludes forming a first oxide material above a substrate and forming asecond oxide material above the first oxide material to form amultilayer dielectric coating, wherein the second oxide material is on aside of the multilayer dielectric coating for contacting an alkali.

Other aspects and embodiments of the present invention will becomeapparent from the following detailed description, which, when taken inconjunction with the drawings, illustrate by way of example theprinciples of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a diagram of energy levels involved in lasing a rubidiumline in an alkali laser, according to the prior art.

FIG. 2 shows a schematic diagram of a diode pumped alkali gas laser,according to one embodiment.

FIG. 3 shows an electrochemical series.

FIGS. 4A-4B show coatings that are resistant to alkali attack withcorresponding plots of reflectivity (%R), according to severalembodiments.

FIG. 5 shows a schematic diagram of a multilayer dielectric coating,according to one embodiment.

FIG. 6 shows schematic diagrams of several structure having interiorsand a multilayer dielectric coating, according to various embodiments.

FIG. 7 is a flow diagram of a method for forming an alkali resistantcoating according to one embodiment.

DETAILED DESCRIPTION

The following description is made for the purpose of illustrating thegeneral principles of the present invention and is not meant to limitthe inventive concepts claimed herein. Further, particular featuresdescribed herein can be used in combination with other describedfeatures in each of the various possible combinations and permutations.

Unless otherwise specifically defined herein, all terms are to be giventheir broadest possible interpretation including meanings implied fromthe specification as well as meanings understood by those skilled in theart and/or as defined in dictionaries, treatises, etc.

It must also be noted that, as used in the specification and theappended claims, the singular forms “a,” “an” and “the” include pluralreferents unless otherwise specified.

In one general embodiment, a multilayer dielectric coating for use in awindow of an alkali laser includes two or more alternating layers ofhigh and low index dielectric materials, wherein an innermost layer ofthe two or more alternating layers of high and low index dielectricmaterials includes at least one of: alumina, zirconia, and hafnia forprotecting subsequent layers of the two or more alternating layers ofhigh and low index dielectric materials from alkali attack.

In another general embodiment, a method for forming an alkali resistantcoating includes forming a first oxide material above a substrate andforming a second oxide material above the first oxide material to form amultilayer dielectric coating, wherein the second oxide material is on aside of the multilayer dielectric coating for contacting an alkali.

As shown in FIG. 2, which shows a schematic diagram of an alkali laser200, the laser 200 may be end pumped, but this requires guiding the pumplight down a suitable tube 202 with a highly reflective coating thatserves as a light guide. This gas cell 202 will contain the alkali gas,which may attack the walls of the gas cell 202.

The high reactivity and the very electropositive nature of the alkaliatoms require that special attention be paid to all the materials thatare in contact with the vapor. For example, metals that otherwise mightserve as reflective walls or wall coatings for the laser light guide maynot be used. Gold and silver will readily form an amalgam with thealkali metal and lose their high reflectivity. Other metals that mightbe compatible with alkalis, such as niobium, tungsten, or other esotericmetals do not have nearly high enough reflectivity to serve as lightguides. Also, commonly used dielectric stack materials, such as tantala(Ta₂O₅) and niobia (Nb₂O₅), both which have a high index of refraction,may not be used as they will be readily reduced by the alkali vapor.Silica, which has a low index of refraction, is also problematic sincealkalis are known to diffuse through thin silica coatings into the opennetwork structure of vitreous silica, rendering them unusable.

In addition, rubidium in the alkali laser is highly corrosive andreduces many materials. Polycrystalline alumina (Al₂O₃) (PCA) is notreduced by alkalis and is used to contain alkalis, such as in highpressure sodium lamps. Zirconia (ZrO₂) is close to alumina in theelectrochemical series shown in FIG. 3, and is also not easily reduced.The electrochemical series in FIG. 3 indicates the relative ease withwhich elements and compounds may be reduced, with entries near the topof the series being more difficult to reduce than entries near thebottom. Tantala and niobia, for example, two high index materials, aremuch more easily reduced than is zirconia. Alumina and zirconia are nearthe top of the series shown in FIG. 3 which indicates that while theycan be reduced by the more reactive alkalis free energy driving force,this reaction is less likely than for other potential oxide couples thatare commonly used for dielectric optical coatings.

Since the possibility of an alkali laser was proposed (see Beach et al.“End-pumped continuous-wave alkali vapor lasers: experiment, model, andpower scaling,” J. Opt. Soc. Am. B., Vol. 12, December 2004), there hasbeen considerable interest in this new class of high power lasers. Theselasers have many benefits, such as being able to be pumped by commerciallaser diode bars, increased efficiency because they use a small amountof alkali vapor in an inert buffer gas (such as helium), and the abilityto produce beams with little wave front distortion. However, producing acoating which may be used with these lasers has been problematic, asdiscussed previously.

A coating may be made highly transmissive for end windows and/or highlyreflective for pump light guides, and the design of these multilayercoating stack structures is known in the state of the art. However,because of the highly reactive alkali vapors used as the lasing medium,the optical coatings must be made of materials that will not bechemically or physically attacked by the vapor. This is difficult.Metals commonly used for light guides, such as gold and silver, quicklyamalgamate with the alkali and their reflectivity is rapidly degraded.Similarly, many dielectric coating materials will be quickly reduced bythe highly electropositive alkali.

According to one embodiment, alumina, which is compatible with alkalis,may be used as the innermost layer in a dielectric stack coating for analkali laser. The term innermost is hereinafter used to refer to thelayer furthest from the substrate and in direct contact with the alkalicontaining vapor. Hence, the innermost alumina layer is exposed to thealkali vapor, and is capable of protecting any subsequent (outer) layersas long as the alumina layer has a sufficient thickness and density.Alumina is resistant to alkali vapors as evidenced by its use, forexample, in high pressure sodium street lamps that contain sodium vaporat high temperatures hermetically sealed inside translucent alumina arctubes. According to various embodiments, these optical coatings may bedesigned using multilayer interference films of alumina and zirconia,preferably with alumina as the innermost coating.

In addition, in further embodiments, methods for producing thesecoatings are presented. Commonly used vapor deposition techniques do notproduce sufficiently dense films and often absorb water. Two techniques:atomic layer deposition (ALD) and ion beam sputtering (IBS) can producethe alkali resistant coatings, according to several embodiments. ALD iscommonly used to produce alumina coatings to protect solar panels frommoisture. It has been found that an ALD alumina coating of sufficientthickness (see embodiments below) will also protect against alkaliattack. While IBS is conventionally used for many applications, it isnot known to use this method of deposition to produce films that areimpermeable to alkali vapors and do not take up water, which wouldimmediately react with the alkali vapor and defeat the coatings.

Accordingly, in one embodiment, a coating includes alumina and/orzirconia in a suitable dielectric stack multilayer for a highlytransmissive and highly reflective coating. Additionally, a method forproducing the coating is presented in another embodiment, which includesALD and/or IBS. The resultant coatings are found to produce robustcoatings for alkali lasers.

Now referring to FIGS. 4A-4B, coatings that are resistant to attack byalkali vapor, such as in an alkali vapor laser, are shown, along withcorresponding plots of reflectivity (% R). In FIG. 4A, the coatingdesign 400 may be used as a high transmission optical window (ARcoating) On the end of the laser, and for high reflectivity coatings (HRcoatings) along the sides of the pump light guide.

In FIG. 4B, an antireflection coating (AR) 450 is shown that may be usedto reduce reflection of laser light. Referring again to FIGS. 4A-4B, theantireflection coatings (AR) 450 and/or highly reflective coatings (HR)400 may be produced using alumina (Al₂O₅, AlO_(x)), zirconia (ZrO₂),tantala (Ta₂O₅, TaO_(x)), hafnia (HfO₂), silica (SiO₂), niobia(NbO_(x)), magnesium oxide (MgO_(x)) beryllium oxide (BeO), etc.,according to various embodiments. The 402 layers preferably comprisezirconia, and the 404 layers preferably comprise alumina. Alumina maypreferably be used as the innermost layer so that it is in contact withthe alkali vapor, while the composition of the subsequent layers may bedetermined based on desired properties of the coating. The sketches andthe graphs of reflectance next to the layer schematics are forillustrative purposes only, and not meant to be limiting in any way. Thenumber of layers, thicknesses of layers, and/or performancecharacteristics may depend on the characteristics and properties of thestack.

In one embodiment, the number of layers and materials of constructionmay be determined using an optimization computer code as would beapparent to one of skill in the art upon reading the presentdescriptions. Design parameters may also depend on the specificationsfor laser operation. Any AR and HR optical designs employing any ofthese materials (zirconia, alumina, tantala, hafnia, silica, niobia,magnesium oxide, beryllium oxide, etc.) may be produced that are robustto alkali vapor environments. In addition, materials other than thoselisted above may be used, in various amounts and positions in thestacks, as would be understood by one of skill in the art upon readingthe present descriptions as long as the innermost layer is alkaliresistant and impenetrable to the alkali vapors.

In addition to having a robust coating design, the method of depositingthe coatings should provide a very dense coating, one that is not porousand does not absorb water. It is important, in one embodiment, that thealumina and zirconia layers be deposited using atomic layer deposition(ALD) or ion beam sputtering (IBS) of oxide targets or some othercoating technique capable of producing very dense coatings, as would beknown in the art. ALD and IBS are known to produce coatings that areimpervious and actually protect any substrate or subsequent layersbelow. Chemical vapor deposition (CVD) does not produce as dense acoating as IBS, but may still be used in some instances.

According to some embodiments, a dielectric multilayer coating comprisesa high index oxide material and a low index oxide material that isresistant to reduction or any other chemical attack by an alkali, suchas one chosen from a group consisting of sodium, potassium, rubidium,and cesium, among others. In some embodiments, this dielectricmultilayer coating may comprise two or more layers of alternating highand low index materials designed to produce either very low reflectance(AR coating) or very high reflectance (HR coating). The specific coatingdesign, that is, a thicknesses of each layer, may be arrived at usingstandard optical coating design algorithms.

Now referring to FIG. 5, according to one embodiment, a multilayerdielectric coating 500 for use in alkali vapor atmospheres 510 includestwo or more alternating layers of a higher index oxide material 502 anda lower index oxide material 504. The alternating layers may be formedabove a substrate 512, in some approaches.

In one approach, the multilayer dielectric coating 500 may producereflectance of less than about 5% at an angle of incidence of about 0°for a wavelength of light of between about 650 nm and about 900 nm,e.g., the coating is an AR coating. In an alternate approach, themultilayer dielectric coating 500 may produce reflectance of greaterthan about 98% at a wavelength of between about 650 nm and about 900 nmof light having an angle of incidence of between about 50° and about90°, e.g., the coating is an HR coating. Of course, other ranges andreflectances are possible, depending on the choice of the alkali, thespecified transmissions and/or reflectances, the materials, thicknesses,and numbers of alternating layers, etc., and a multilayer dielectriccoating 500 may be designed as desired as would be apparent to one ofskill in the art upon reading the present descriptions.

In one embodiment, an innermost layer 506 of the two or more alternatinglayers may protect subsequent layers 508 from alkali attack. Thisinnermost layer 506 may comprise at least one of alumina, zirconia, andhafnia; and more preferably, it may comprise alumina. According to afurther embodiment, the subsequent layers 508 of the two or morealternating layers may comprise alternating layers of at least two of:zirconia, alumina, tantala, hafnia, silica, niobia, magnesium oxide, orberyllium oxide.

In a preferred embodiment, the higher index oxide material 502 may bechosen from: zirconia, hafnia, tantala, and niobia, among others. Inanother preferred embodiment, the lower index oxide material 504 may bechosen from: alumina, silica, zirconia, hafnia, magnesium oxide, andberyllium oxide, among others. The index of the materials chosen arehigher or lower in relation to each other, and not necessarily to anystandard or baseline index value.

The choice of the subsequent layers 508 may be based on cost, ease ofproduction, reflectance of the finished coating, etc. Preferably, twoadjacent layers are not formed of the same material, e.g., the layersform alternating layers of higher and lower index materials.

According to additional embodiments, a thickness of the innermost layer506 may be between about 500 nm and about 1000 nm, and in anotherembodiment designed for harsher alkali atmospheres, the thickness of theinnermost layer 506 may be greater than about 1000 nm.

In a more specific embodiment, a multilayer dielectric coating 500 mayinclude alumina, zirconia, and/or hafnia, as an innermost layer 506 ofthe multilayer coating. A preferred embodiment includes alumina as theinnermost layer. The innermost layer is the layer adjacent to the vaporhaving the alkali in an alkali laser. In this way, the alkali resistantlayer is operable to protect other layers of the multilayer coating,along with other materials, such as a substrate, in the alkali laser orother device that contacts an alkali vapor, from attack by the alkali.In some approaches, the innermost layer may be at least about 250 nmthick, more preferably at least about 500 nm thick, and even morepreferably greater than about 1000 nm thick for use in harsher alkaliatmospheres.

In more approaches, the alternating layers may be formed above asubstrate 512 positioned on a non-alkali contacting side of themultilayer dielectric coating 500. The substrate may be formed of anymaterial known in the art, such as sapphire, fused silica, glass,ceramic alumina, etc.

Now referring to FIG. 6, several structures 600, 610, 620 are shown,each structure having an interior 602 and a dielectric coating,according to various embodiments. The structures 600, 610, 620 are notshown to scale, and in most applications, the interior 602 of thestructure may be much larger than shown in the figures, and thethicknesses and number of the layers may be different than shown. Thesubstrate 512 may define this structure 600 having a cylindrical or ovalcross-section, or any other structure, such as one having a rectangularcross-section 610, a polygonal cross-section, an irregular polygonalcross-section 620, etc., according to various embodiments. In any ofthese embodiments, the higher index oxide material 502 and the lowerindex oxide material 504 may be concentric layers in the interior 602 ofthe structure (as shown for the cylindrical structure 600). In allcases, the innermost layer 506 provides protection to subsequent layersfrom alkali attack when an alkali vapor 510 is present in the interior602, and may be formed thicker than the subsequent layers, as shown inthe figures.

Also, the lower index oxide material 504 may be an innermost layer 506of the concentric layers, and the lower index oxide material 504 maypreferably comprise alumina, in one embodiment. However, layers of thelower index oxide material 504 other than the innermost layer 506 maycomprise any other low index oxide material, such as alumina, silica,magnesium oxide, etc.

In another preferred embodiment, the innermost layer 506 may have adensity of greater than about 97% of its theoretical density, i.e., thedensity of the crystalline material, whether it be alumina, zirconia, orhafnia. This density may be achieved by using ALD or IBS formationmethods, among others. The high density of the innermost layer enablesprotection of the other layers and a substrate from attack by the alkalivapor.

In more embodiments, other multilayer pair options for optical coatingsare possible, such as silica and hafnia, silica and alumina, silica andzirconia, etc., and may alternatively be used as long as alumina,zirconia or hafnia is applied as the innermost layer 506, as describedabove.

In one embodiment, the innermost layer 506 of the concentric layerscomprising alumina may have a density of greater than about 97% of atheoretical density of alumina.

According to further embodiments, any of the structures 600, 610, 620may form a gas cell, and may contain, hold, surround, and/or include analkali vapor and a buffer gas within the structure 600, 610, 620,thereby forming, as shown in FIG. 2, an alkali vapor and buffer gas cell202. Additionally, as shown in FIG. 2, a dot reflector 212 may bepositioned on an end of the alkali vapor and buffer gas cell 202 forforming a laser beam 204. Any of the other associated components asshown in FIG. 2 may also be included in an alkali gas laser 200, such asa vessel 206 for holding the alkali metal, a hollow lens duct 208, aradiance conditioned diode array 210, etc.

In another embodiment, an alkali gas laser 200 may include an alkalivapor and a buffer gas within a gas cell 202, wherein walls of the gascell 202 comprise a multilayer dielectric coating as described hereinaccording to any embodiment. Additionally, the alkali gas laser 200 mayinclude a (HR) dot reflector 212 for forming a laser beam 204.

In an even more preferred embodiment, the entirety of the multilayerdielectric coating may comprise alkali resistant materials, such as analkali resistant material in a single layer coating, a multilayercoating comprising two alkali resistant materials, etc. In one suchembodiment, the alkali resistant multilayer coating may comprise aluminaand zirconia in alternate or alternating layers, comprising two layers,three layers, four layers, ten layers, 100 layers, etc. In this case,although both materials are resistant to alkali attack, the innermostlayer may comprise the most alkali resistant material, alumina, and itmay be applied as discussed above, e.g., through IBS.

Now referring to FIG. 7, a method 700 for forming an alkali resistantcoating is shown according to one embodiment. The method 700 may becarried out in any desired environment. Of course, the method 700 mayinclude more or less operations than those shown in FIG. 7, as would beapparent to one of skill in the art upon reading the presentdescriptions.

In operation 702, a first oxide material is formed above a substrate.Any formation method may be used, as known in the art, such as IBS, CVD,etc.

In operation 704, a second oxide material is formed above the firstoxide material to form a multilayer dielectric coating. The second oxidematerial is on a side of the multilayer dielectric coating forcontacting an alkali, when the coating is in use.

In one embodiment, additional alternating layers of the first oxidematerial and the second oxide material may be formed that number morethan 10 total layers, more than 100 layers, more than 300 layers, morethan 1000 layers, etc.

In another embodiment, the multilayer dielectric coating may producereflectance of greater than about 98% at a 780 nm wavelength of lighthaving an angle of incidence of between about 50° and about 90°, e.g.,the coating is an HR coating.

In another embodiment, the multilayer dielectric coating may producereflectance of less than about 5% at an angle of incidence of about 0°for a wavelength of light of between about 650 nm and about 900 nm,e.g., the coating is an AR coating.

In another embodiment, the substrate may define a structure having aninterior. In this embodiment, the first oxide material and the secondoxide material may form concentric layers in the interior of thestructure, and the second oxide material is an innermost layer of theconcentric layers, thereby allowing it to protect subsequent layers fromattack from an alkali vapor that may be included in the interior. In afurther embodiment, the innermost layer of the concentric layers mayinclude at least one of: alumina, zirconia, and hafnia, among otheralkali resistant materials.

In another embodiment, the innermost layer of the concentric layers mayinclude alumina, and a density of the innermost layer is greater thanabout 97% of a theoretical density of alumina.

According to one approach, the subsequent layers may include alternatinglayers of at least two of alumina, zirconia, tantala, hafnia, or silica,preferably in an arrangement where a high index oxide material isalternated with a low index oxide material.

In more approaches, the innermost layer of the concentric layers may beformed to a thickness of greater than about 250 nm, 1000 nm, etc.

In another embodiment, the substrate may include, but is not limited to,any one of sapphire, fused silica, glass, ceramic alumina, or anycombination thereof.

In a preferred embodiment, at least the second oxide material may beformed via ion beam sputtering, or any other method capable ofdepositing a highly dense structure, e.g., with a density of greaterthan about 97% of a theoretical density of the material.

While various embodiments have been described above, it should beunderstood that they have been presented by way of example only, and notlimitation. Thus, the breadth and scope of a preferred embodiment shouldnot be limited by any of the above-described exemplary embodiments, butshould be defined only in accordance with the following claims and theirequivalents.

1. A multilayer dielectric coating for use in a window or reflector ofan alkali laser, the coating comprising two or more alternating layersof high and low index dielectric materials, wherein an innermost layerof the two or more alternating layers of high and low index dielectricmaterials comprises at least one of: alumina, zirconia, and hafnia forprotecting subsequent layers of the two or more alternating layers ofhigh and low index dielectric materials from alkali attack.
 2. Themultilayer dielectric coating as recited in claim 1, wherein the coatingis an anti-reflection (AR) coating and produces reflectances of lessthan about 5% at an angle of incidence for a laser beam having awavelength of between about 650 nm and about 900 nm.
 3. The multilayerdielectric coating as recited in claim 1, wherein the coating is a highreflection (HR) coating for pump light for alkali laser wave guidewalls, wherein the coating produces reflectances of greater than about98% at pump wavelengths having an angle of incidence of between about50° and about 90°.
 4. The multilayer dielectric coating as recited inclaim 1, wherein the innermost layer of the two or more alternatinglayers of high and low index dielectric materials is specificallydesigned so as to resist reduction and damage caused by an alkali vaporwithin the alkali laser.
 5. The multilayer dielectric coating as recitedin claim 1, wherein a thickness of the innermost layer is between about500 nm and about 1000 nm.
 6. The multilayer dielectric coating asrecited in claim 1, wherein a thickness of the innermost layer isgreater than about 1000 nm.
 7. The multilayer dielectric coating asrecited in claim 1, further comprising a substrate on a non-alkalicontacting side of the multilayer dielectric coating, wherein thesubstrate comprises one of: sapphire, polycrystalline alumina, fusedsilica, glass, niobium, stainless steel, or an iron/nickel alloy.
 8. Themultilayer dielectric coating as recited in claim 1, wherein theinnermost layer has a density of greater than about 97% of a theoreticaldensity.
 9. The multilayer dielectric coating as recited in claim 1,wherein the innermost layer is deposited via at least one of: atomiclayer deposition (ALD) that employs suitable organometallic compoundsthat decompose on a surface to produce a single atomic layer, ion beamsputtering (IBS) that uses ions to sputter material from a target anddeposit the sputtered target material on a surface, and ion beamassisted vapor deposition.
 10. The multilayer dielectric coating asrecited in claim 1, wherein the innermost layer comprises alumina andthe subsequent layers comprise alternating layers of at least two of:alumina, zirconia, tantala, niobia, hafnia, magnesium oxide, berylliumoxide, and silica.
 11. The multilayer dielectric coating as recited inclaim 10, wherein a thickness of the innermost layer is between about500 nm and about 1000 nm.
 12. The multilayer dielectric coating asrecited in claim 10, wherein a thickness of the innermost layer isgreater than about 1000 nm.
 13. The multilayer dielectric coating asrecited in claim 10, further comprising a substrate on a non-alkalicontacting side of the multilayer dielectric coating, wherein thesubstrate comprises one of: sapphire, polycrystalline alumina, fusedsilica, glass, niobium, stainless steel, or an iron/nickel alloy. 14.The multilayer dielectric coating as recited in claim 10, wherein theinnermost layer of the two or more alternating layers of high and lowindex dielectric materials has a density of greater than about 97% of atheoretical density.
 15. The multilayer dielectric coating as recited inclaim 10, wherein the innermost layer is deposited via at least one of:atomic layer deposition (ALD) that employs suitable organometalliccompounds that decompose on a surface to produce a single atomic layer,ion beam sputtering (IBS) that uses ions to sputter material from atarget and deposit the sputtered target material on a surface, and ionbeam assisted vapor deposition.
 16. The multilayer dielectric coating asrecited in claim 1, wherein the innermost layer comprises alumina andthe subsequent layers comprise alternating layers of: alumina andzirconia.
 17. The multilayer dielectric coating as recited in claim 16,wherein a thickness of the innermost layer is between about 500 nm andabout 1000 nm.
 18. The multilayer dielectric coating as recited in claim16, wherein a thickness of the innermost layer is greater than about1000 nm.
 19. The multilayer dielectric coating as recited in claim 16,further comprising a substrate on a non-alkali contacting side of themultilayer dielectric coating, wherein the substrate comprises one ofsapphire, polycrystalline alumina, fused silica, glass, niobium,stainless steel, or an iron/nickel alloy.
 20. The multilayer dielectriccoating as recited in claim 16, wherein the innermost layer of the twoor more alternating layers of high and low index dielectric materialshas a density of greater than about 97% of a theoretical density. 21.The multilayer dielectric coating as recited in claim 16, wherein theinnermost layer is deposited via at least one of: atomic layerdeposition (ALD) that employs suitable organometallic compounds thatdecompose on a surface to produce a single atomic layer, ion beamsputtering (IBS) that uses ions to sputter material from a target anddeposit the sputtered target material on a surface, and ion beamassisted vapor deposition.
 22. A method for forming an alkali resistantcoating, the method comprising: forming a first oxide material above asubstrate; and forming a second oxide material above the first oxidematerial to form a multilayer dielectric coating, wherein the secondoxide material is on a side of the multilayer dielectric coating forcontacting an alkali.
 23. The method as recited in claim 22, wherein thealkali resistant coating produces reflectances of less than about 5% atan angle of incidence for a laser beam having a wavelength of betweenabout 650 nm and about 900 nm.
 24. The method as recited in claim 22,wherein the alkali resistant coating produces reflectances of greaterthan about 98% at pump wavelengths having an angle of incidence ofbetween about 50° and about 90°.
 25. The method as recited in claim 22,wherein the substrate defines a structure having an interior, whereinthe first oxide material and the second oxide material form concentriclayers in the interior of the structure, and wherein the second oxidematerial is an innermost layer of the concentric layers.
 26. The methodas recited in claim 25, wherein the innermost layer of the concentriclayers protects subsequent layers from alkali attack, the innermostlayer of the concentric layers comprising at least one of: alumina,zirconia, and hafnia.
 27. The method as recited in claim 26, wherein theinnermost layer of the concentric layers comprises alumina, and whereina density of the innermost layer is greater than about 97% of atheoretical density of alumina.
 28. The method as recited in claim 26,wherein the subsequent layers comprise alternating layers of at leasttwo of: alumina, zirconia, tantala, niobia, hafnia, magnesium oxide,beryllium oxide, and silica.
 29. The method as recited in claim 26,wherein the innermost layer of the concentric layers is formed to athickness of greater than about 500 nm.
 30. The method as recited inclaim 26, wherein the innermost layer is deposited via at least one ofatomic layer deposition (ALD) that employs suitable organometalliccompounds that decompose on a surface to produce a single atomic layer,ion beam sputtering (IBS) that uses ions to sputter material from atarget and deposit the sputtered target material on a surface, and ionbeam assisted vapor deposition.
 31. The method as recited in claim 22,wherein the substrate comprises one of: sapphire, polycrystallinealumina, fused silica, glass, niobium, stainless steel, or aniron/nickel alloy.